This invention relates to a composition that is advantageously used as a matrix or a binder in the production of composite carbon materials. The invention also relates to a composite carbon material that is produced using that composition, as well as a process for producing that composite carbon material.
Composite carbon materials hold much promise as braking materials in aircraft and automobiles, structural materials of nuclear reactors, spacecraft and aircraft, and as corrosion-resistant or sliding materials in various industries. In this regard, it is worth particular mentioning that methods are being studied today for producing high-performance composite carbon materials that are improved in strength and other characteristics using mesophase pitch as a binder or matrix in place of heretofore used phenolic resins, furan resins and tar pitch.
However, the yield of carbonization of binder or matrix mesophase pitch which is used with an aggregate in the production of various composite carbon materials is not satisfactorily high, and gases or volatiles that occur in the process of heat treatment will produce fine bubbles in the product, causing a decrease in its density and mechanical strength. This makes it necessary for the impregnation with pitch and the carbonization at high temperature to be repeated several times. As a further problem, pitch will often flow out of the shaped part in the process of carbonization, causing the product to deform and lose dimensional stability.
The process of manufacturing composite carbon materials comprises either heating under pressure up to a temperature of ca. 600.degree. C. at which pitch turns to coke or carbonizing at a very small rate of temperature elevation not exceeding 1.degree. C./min. Hence, the production process of composite carbon materials is not only complicated but also costly because of the long time needed to obtained the final product.
Various attempts have recently been made to solve the aforementioned problems of the art. See, for example, Japanese Patent Public Disclosure No. 212275/1989, which describes a process for producing a carbon-fiber reinforced composite carbon material that comprises impregnating a fiber-preformed part with carbonaceous pitch and performing stabilization several times in an oxidizing gas atmosphere, followed by carbonization and graphitization. Also see Japanese Patent Public Disclosure No. 239060/1989, which proposes that fine stabilized spheres of mesophase pitch having a random configuration of optical anisotropic structures should be used as a matrix for carbon-fiber reinforced composite carbon materials.
Those methods are both intended to prevent the formation of bubbles, dislodging of the matrix and the deformation of a shaped part which occur on account of the melting or flowing out of a pitch or the evolution of gases in the process of carbonization. However, the method proposed by Japanese Patent Public Disclosure No. 212275/1989 has the disadvantage of being costly since it is necessary to repeat the cycles of stabilization and cooling. The proposal made by Japanese Patent Public Disclosure No. 239060/1989 is based on the use of a stabilized pitch, so the degree of pitch impregnation is insufficient to produce a product of high performance.
It has also been attempted to solve the problems at issue by adding a polymerization accelerator to the pitch. See, for example, U.S. Pat. No. 2,500,208 and 2,500,209, which propose the use of an aromatic nitro compound as a polymerization accelerator. "Tanso (Carbon)", No. 119, pp. 190-196, 1984 describes the preparation of a sample of shaped carbon part using a sulfur-doped pitch.
In certain cases, the methods proposed by the U.S. patents, supra, achieve an appreciably high degree of effectiveness in promoting the polymerization of pitch. However, it is difficult to inhibit the formation of bubbles adequately in the heat-treatment step and this makes it impossible to increase the rate of temperature elevation.
In the method described in "Tanso", ibid., no more than 5% of sulfur was added to pitch softening at 185.degree. C. and a heat treatment was conducted at 450.degree. C. to increase the softening point to 320.degree. C., and the resulting sample was molded and heat-treated. The relationship between the evolution of H.sub.2 S and the temperature was such that comparatively satisfactory results were obtained only when extremely thin-walled shaped parts were heat-treated at a very slow heating rate of 0.1.degree. C./min. A binder or matrix pitch is shaped together with an aggregate and other components of a composite carbon material to be produced but in the method proposed by "Tanso", ibid., the pitch was too viscous to be thoroughly impregnated in interstices in the aggregate, thus making it impossible to attain high mechanical strength.
As described above, various studies have so far been made with a view to producing a variety of high-performance composite carbon materials at low cost. However, none of them are yet to be completely satisfactory and composite carbon materials have to be produced by a complicated process over a prolonged time.